1. Field of the Invention
The invention relates to mixing methods for mixing components in a mixing vessel in alternate directions.
2. Related Art
In industrial mixing equipment, the geometry of a mixing vessel and the design of a mixing impeller provided in the mixing vessel provide a wide range of flow behaviors when mixing components in the mixing vessel. If the mixing vessel is not adequately baffled, a tangential “swirling” motion dominates and axial “up and down” flow is suppressed. In the worst case, the components in the mixing vessel may move as a single body. This condition, which is marked by a strong central vortex, is actually known by researchers to be detrimental for several reasons. Air, which is ingested into the mixing vessel, can introduce dangerous instability into the rotating mixing impeller. The central vortex can actually prevent floating solids from being incorporated into the bulk. The increased air/liquid interface can damage sensitive molecules, for example proteins.
In conventional engineering practice, it is recommended that mixing vessels are baffled to eliminate the swirling tangential motion and thereby suppress the central vortex. Baffles generally take the form of narrow plates that extend outward from a mixing vessel wall. In most mixing vessels, for example stainless mixing vessels, the addition of such baffles are economical and practical.
In a single-use mixing vessel, like a flexible single-use bioreactor, however, the addition of rigid baffles is cumbersome. Rigid baffles complicate the folding of empty bags. In addition, the flexible walls of the single-use mixing vessel do not offer a convenient support structure for rigid baffles. One approach which has been adopted by several companies is to use a square or rectangular container as a mixing vessel. The corners of such a rectangular mixing vessel behave like virtual baffles, interrupting the swirling tangential flow and promoting axial flow. However, it is not often possible to achieve a perfect 90 degrees angle at all corners of the single-use container. Since the tolerances on bag dimensions are generally much larger than the tolerances on rigid box dimensions, it may be that the bag is intentionally undersized compared to its rigid support structure to ensure that there is not excess material that would pose a challenge during installation or filling of the bag. Undersizing the bag results in rounding at the corners, and this rounding at the corners has been shown to promote swirling tangential flow.
Therefore, it is desired to prevent the swirling tangential flow and promote axial flow in unbaffled cylindrical mixing vessels and for square or rectangular mixing vessels.